Summary. Stereological methods were employed in two experiments with adult stallions: (1) to confirm seasonal variation in number of Sertoli cells and (2) to characterize the annual cycle of the Sertoli cell population. In the first experiment, testes from 28 adult (4–20 years old) horses obtained in the non-breeding season (December—January) were compared to testes from 28 adult horses in the breeding season (June—July). Sertoli cell numbers were calculated from the nuclear volume density, parenchymal volume, and volume of an individual Sertoli cell nucleus determined by reconstruction of serial sections or from average height and width measurements. The number of Sertoli cells per testis was significantly greater in the breeding season. In a second experiment involving 43–48 adult horses in each 3-month period, the Sertoli cell population was higher (P < 0.05) in May—July than other periods and higher (P < 0.01) than in November—January. These combined studies confirm seasonal differences in the Sertoli cell numbers per testis and define the annual cycle of the Sertoli cell population in adult stallions.